In 1989, an endotoxin-induced serum activity that induced interferon-γ (IFN-γ) obtained from mouse spleen cells was described (Nakamura et al., 1989). This serum activity functioned not as a direct inducer of IFN-γ but rather as a co-stimulant together with IL-2 or mitogens. An attempt to purify the activity from post-endotoxin mouse serum revealed an apparently homogeneous 50-55 kDa protein. Since other cytokines can act as co-stimulants for IFN-γ production, the failure of neutralizing antibodies to IL-1, IL4, IL-5, IL-6, or TNF to neutralize the serum activity suggested it was a distinct factor. In 1995, the same scientists demonstrated that the endotoxin-induced co-stimulant for IFN-γ production was present in extracts of livers from mice preconditioned with P. acnes (Okamura et al., 1995). In this model, the hepatic macrophage population (Kupffer cells) expand and in these mice, a low dose of bacterial lipopolysaccharide (LPS), which in non-preconditioned mice is not lethal, becomes lethal. The factor, named IFN-γ-inducing factor (IGIF) and later designated interleukin-18 (IL-18), was purified to homogeneity from 1,200 grams of P. acnes-treated mouse livers. Degenerate oligonucleotides derived from amino acid sequences of purified IL-18 were used to clone a murine IL-18 cDNA. IL-18 is an 18-19 kDa protein of 157 amino acids, which has no obvious similarities to any peptide in the databases. Messenger RNAs for IL-18 and interleukin-12 (IL-12) are readily detected in Kupffer cells and activated macrophages. Recombinant IL-18 induces IFN-gamma more potently than does IL-12, apparently through a separate pathway (Micallef et al., 1996). Similar to the endotoxin-induced serum activity, IL-18 does not induce IFN-γ by itself, but functions primarily as a co-stimulant with mitogens or IL-2. IL-18 enhances T cell proliferation, apparently through an IL-2-dependent pathway, and enhances Th1 cytokine production in vitro and exhibits synergism when combined with IL-12 in terms of enhanced IFN-γ production (Maliszewski et al., 1990).
After the murine form was cloned, the human cDNA sequence for IL-18 was reported in 1996 (Ushio et al., 1996).
By cloning IL-18 from affected tissues and studying IL-18 gene expression, a close association of this cytokine with an autoimmune disease was found. The non-obese diabetic (NOD) mouse spontaneously develops autoimmune insulitis and diabetes, which can be accelerated and synchronized by a single injection of cyclophosphamide. IL-18 mRNA was demonstrated by reverse transcriptase PCR in NOD mouse pancreas during early stages of insulitis. Levels of IL-18 mRNA increased rapidly after cyclophosphamide treatment and preceded a rise in IFN-γ mRNA, and subsequently diabetes. Interestingly, these kinetics mimic that of IL-12-p40 mRNA, resulting in a close correlation of individual mRNA levels. Cloning of the IL-18 cDNA from pancreas RNA followed by sequencing revealed identity with the IL-18 sequence cloned from Kupffer cells and in vivo pre-activated macrophages. Also NOD mouse macrophages responded to cyclophosphamide with IL-18 gene expression while macrophages from Balb/c mice treated in parallel did not. Therefore, IL-18 expression is abnormally regulated in autoimmune NOD mice and closely associated with diabetes development (Rothe et al., 1997).
IL-18 plays a potential role in immunoregulation or in inflammation by augmenting the functional activity of Fas ligand on Th1 cells (Conti et al., 1997). IL-18 is also expressed in the adrenal cortex and therefore might be a secreted neuro-immunomodulator, playing an important role in orchestrating the immune system following a stressful experience (Chater, 1986).
In vivo, IL-18 is formed by cleavage of pro-IL-18, and its endogenous activity appears to account for IFN-γ production in P. acnes and LPS-mediated lethality. Mature IL-18 is produced from its precursor by the IL-1β converting enzyme (IL-1beta-converting enzyme, ICE, caspase-1).
The IL-18 receptor consists of at least two components, co-operating in ligand binding. High- and low-affinity binding sites for IL-18 were found in murine IL-12 stimulated T cells (Yoshimoto et al., 1998), suggesting a multiple chain receptor complex. Two receptor subunits have been identified so far, both belonging to the IL-1 receptor family (Parnet et al., 1996). The signal transduction of IL-18 involves activation of NF-κB (DiDonato et al., 1997).
Recently, a soluble protein having a high affinity for IL-18 has been isolated from human urine, and the human and mouse cDNAs were described (Novick et al., 1999; WO 99/09063). The protein has been designated IL-18 binding protein (IL-18BP).
IL-18BP is not the extracellular domain of one of the known IL18 receptors, but a secreted, naturally circulating protein. It belongs to a novel family of secreted proteins. The family further includes several Poxvirus-encoded proteins which have a high homology to IL-18BP (Novick et al., 1999). IL-18BP is constitutively expressed in the spleen, belongs to the immunoglobulin superfamily, and has limited homology to the IL-1 type II receptor. Its gene was localized on human chromosome 11q13, and no exon coding for a transmembrane domain was found in an 8.3 kb genomic sequence (Novick et al., 1999).
Four human and two mouse isoforms of IL-18BP, resulting from mRNA splicing and found in various cDNA libraries and have been expressed, purified, and assessed for binding and neutralization of IL-18 biological activities (Kim et al., 2000). Human IL-18BP isoform a (IL-18BPa) exhibited the greatest affinity for IL-18 with a rapid on-rate, a slow off-rate, and a dissociation constant (K(d)) of 399 pM. IL-18BPc shares the Ig domain of IL-18BPa except for the 29 C-terminal amino acids; the K(d) of IL-18BPc is 10-fold less (2.94 nM). Nevertheless, IL-18BPa and IL-18BPc neutralize IL-18>95% at a molar excess of two. IL-18BPb and IL-18BPd isoforms lack a complete Ig domain and lack the ability to bind or neutralize IL-18. Murine IL-18BPc and IL-18BPd isoforms, possessing the identical Ig domain, also neutralize >95% murine IL-18 at a molar excess of two. However, murine IL-18BPd, which shares a common C-terminal motif with human IL-18BPa, also neutralizes human IL-18. Molecular modelling identified a large mixed electrostatic and hydrophobic binding site in the Ig domain of IL-18BP, which could account for its high affinity binding to the ligand (Kim et al., 2000).
Traumatic brain injury (TBI), also simply called head injury, or closed head injury (CHI), refers to an injury of the central nervous system where there is damage to the brain caused by an external blow to the head. It mostly happens during car or bicycle accidents, but may also occur as the result of near drowning, heart attack, stroke and infections. This type of traumatic brain injury would usually result due to the lack of oxygen or blood supply to the brain, and therefore can be referred to as an “anoxic injury”.
Closed head injury occurs when there is a blow to the head as in a motor vehicle accident or a fall. In this case, the skull hits a stationary object and the brain, which is inside the skull, turns and twists on its axis (the brain stem), causing localized or widespread damage. Also, the brain, a soft mass surrounded by fluid that allows it to “float,” may rebound against the skull resulting in further damage.
There may be a period of unconsciousness immediately following the trauma, which may last minutes, weeks or months. Due to the twisting and rebounding, the traumatically brain injured patient usually receives damage or bruising to many parts of the brain. This is called diffuse damage, or “non-missile injury” to the brain. The types of brain damages occurring in non-missile injuries may be classified as either primary or secondary.
Primary brain damage occurs at the time of injury, mainly at the sites of impact, in particular when a skull fraction is present. Large contusions may be associated with an intracerebral haemorrhage, or accompanied by cortical lacerations. Diffuse axonal injuries occur as a result of shearing and tensile strains of neuronal processes produced by rotational movements of the brain within the skull. There may be small heamorrhagic lesions or diffuse damage to axons, which can only be detected microscopically.
Secondary brain damage occurs as a result of complications developing after the moment of injury. They include intracranial hemorrhage, traumatic damage to extracerebral arteries, intracranial herniation, hypoxic brain damage or meningitis.
An “open head injury” is a visible assault to the head and may result from a gunshot wound, an accident or an object going through the skull into the brain (“missile injury to the brain”), This type of head injury is more likely to damage a specific area of the brain.
So called “mild brain injury” may occur with no loss of consciousness and possibly only a dazed feeling or confused state lasting a short time. Although medical care administered may be minimal, persons with brain injury without coma may experience symptoms and impairments similar to those suffered by the survivor of a coma injury.
In response to the trauma, changes occur in the brain, which require monitoring to prevent further damage. The brain's size frequently increases after a severe head injury. This is called brain swelling and occurs when there is an increase in the amount of blood to the brain. Later in the illness water may collect in the brain, which is called brain edema. Both brain swelling and brain edema result in excessive pressure in the brain called intracranial pressure (“ICP”).
Coma is the prolonged period of unconsciousness immediately following the traumatic head injury.
There are several levels of coma. Coma levels can be measured by the progression of responsiveness of the head injured person. In the acute phase of head injury the “Glasgow Coma Scale” is used. As the patient improves or stabilizes, the “Rancho Los Amigos Scale” is used which measures levels of cognitive (understanding and reasoning) thinking.
Brain injury frequently results in persisting debility, such as post-traumatic epilepsy, persistent vegetative state, or post-traumatic dementia.
Spinal cord injury is another type of CNS injury. Spinal cord injuries account for the majority of hospital admissions for paraplegia and tetraplegia. Over 80% occur as a result of road accidents. Two main groups of injury are recognized clinically: open injuries and closed injuries.
Open injuries cause direct trauma of the spinal cord and nerve roots. Perforating injuries can cause extensive disruption and haemorrhage. Closed injuries account for most spinal injuries and are usually associated with a fracture/dislocation of the spinal column, which is usually demonstrable radiologically. Damage to the cord depends on the extent of the bony injuries and can be considered in two main stages: Primary damage, which are contusions, nerve fibre transections and heamorrhagic necrosis, and secondary damage, which are extradural haematoma, infarction, infection and oedema.
Late effects of cord damage include: ascending and descending anterograde degeneration of damaged nerve fibers, post-traumatic syringomelyia, and systemic effects of paraplegia, such as urinary tract and chest infections, pressure sores and muscle wasting.
The pathology of traumatic brain injury is very complex and still poorly understood. Research efforts in the past decade have highlighted an important role of cytokines released systemically and locally within the intrathecal compartment after brain injury, and a dual effect of pro-inflammatory cytokines, such as TNF, IL-6, or IL-8, was hypothesized based on findings of time-dependent beneficial and adverse effects of these mediators (Morganti-Kossmann et al., 1997; Kossmann et al., 1997; Shohami et al., 1999, Scherbel et al., 1999; Whalen et al., 2000). As described above, a recently discovered cytokine of the IL-1 family is IL-18. Recent studies have demonstrated that IL-18 is constitutively expressed in the CNS of mice, rats, and humans in vivo (Culhane et al., 1998; Jander and Stoll, 1998; Prinz et al., 1999; Fassbender et al., 1999; Wheeler et al., 2000), as well as in primary cultures of astrocytes and microglia, but not neurons, in vitro (Conti et al., 1999). Increased IL-18 levels were detected in the cerebrospinal fluid (CSF) of patients with inflammatory CNS diseases, such as bacterial meningitis and viral meningoencephalitis, but not in the CSF of multiple sclerosis (MS) patients (Fassbender et al., 1999). In contrast to the finding of generally low intrathecal IL-18 levels in MS patients, increased IL-18 mRNA expression was demonstrated in spinal cords of Lewis rats with experimental autoimmune encephalomyelitis (EAE), the animal model for MS (Jander and Stoll, 1998). The expression and functional significance of IL-18 in neurotrauma has not been investigated until now.